It is a fair assumption to make, but an important one to correct. Microsoft and Google guarantee the availability of their services, meaning their platforms will stay online, but they do not guarantee the safety of your data. Your data is, and always has been, your responsibility.
According to Microsoft's own shared responsibility model, you are always responsible for your data, your endpoints, your accounts and your access management. If your data is lost due to accidental deletion, a cyber incident or any other cause, Microsoft will not recover it for you. A third-party backup solution creates an independent copy of your data stored separately from the cloud service. If something goes wrong, that backup can be restored. It is the IT equivalent of not keeping all your eggs in one basket, and something we consider essential for any business that takes continuity seriously.